The present invention relates to ultrasonic flow measurement, in particular a device and method for measuring a flow characteristic, e.g. the flow velocity and/or throughput of a fluid in a conduit.
Ultrasonic flow measurement of fluids flowing in a conduit is generally known in the art. Basically it comprises transmitting ultrasonic sound waves between a transmitter and a receiver, that are spaced apart in axial direction of a conduit, in a downstream direction of the fluid flow and in the upstream direction thereof. The transit time of the ultrasonic waves is measured. From the difference in transit time of the downstream directed ultrasonic wave and the upstream directed wave an average flow velocity or throughput can be calculated based on a known geometry of the conduit. The fluid is a gas, vapour or liquid, such as natural gas.
In addition to the flow velocity, the transit time as measured is dependent from the flow profile of the fluid in the conduit. Swirl, cross flow, profile, asymmetry and a velocity fluctuating in time are known distortions from an ideal flow profile, which frequently occur in practice, e.g. in complex piping structures, after bends and the like.
The ultimate reliability of the flow velocity as measured and calculated depends on many parameters, like the distance covered, the acoustic path configuration, transmitted ultrasonic wave type and the calculation method itself. Many acoustic path configurations are known in the art.
One of the known acoustic path configurations is a triangular path having midradius chords, wherein the ultrasonic wave transmitted by the transmitter reflects twice at the inner conduit wall prior to being received by the receiver. E.g. U.S. Pat. No. 5,546,812 has disclosed a method and device for determining characteristics of the flow of a medium in a channel, comprising a transducer arrangement defining two triangular paths offset to each other for swirl determination and three single reflection axial paths also offset to each other for (a) symmetry determination. In commercially available flow meters according to this patent, the first triangular path has a clockwise orientation and the second triangular path has a counter clockwise orientation.
Yet another path configuration known from e.g. EP 0 843 160 A1 comprises at least one first path through the centre of the conduit, at least one second path in the form of an inscribed triangle and at least one third path having three or more reflections against the wall of the conduit. This known path configuration is directed to obtaining a curve of composite weighting factors used in calculating the flow velocity/throughput as close as possible to the ideal weighting factor. It is said that a reduction of the inaccuracy to about 0.15% could be achieved.
The requirements regarding accuracy and reliability of the measured results still increase. Frequently, improving accuracy and reliability if possible at all, is accompanied by a disproportionate increase of complexity and thus costs of the device.